In a conventional discharge chamber of a gas laser system, a pair of electrodes connected to a discharge circuit imparts electrical energy to a gas mixture volume located between the electrodes. The energizing of the gas mixture volume results in an excitation of the atoms and/or molecules in the gas mixture volume. The atoms and/or molecules remain in the excited state only for a very short period of time, i.e., roughly 10−8 seconds and a stimulated emission takes place if certain prerequisites are fulfilled. The stimulated emission leads to the generation of coherent radiation. In order to obtain a highly directionally orientated light beam, the discharge chamber is positioned within a resonator which is an optical feedback system and which usually comprises two mirrors. The mirrors are arranged on opposite sides of the resonator, thus forcing the radiation to oscillate within the resonator. One of the mirrors is totally reflective while the other allows a fraction of the light to escape from the resonator, e.g., through a partially transparent section, thereby forming a laser beam.
Many gas laser systems produce short excitation pulses, leading to laser pulses of 10 ns to 30 ns. For improving the spatial uniformity of the gas discharge, the gas mixture volume is preionized by using, for example, preionization pins. The preionization pins are placed close to the electrodes and generate a spark discharge some 10 ns before the main gas discharge. The sparks produce ultraviolet radiation which is sufficient to preionize the gas mixture volume between the electrodes with a homogeneous initial seed density of about 108 electrons/cm3.
Many applications, for example optical microlithography for forming small electronic structures on silicon substrates, require gas laser systems to be run at high power, while maintaining a necessarily high repetition rate of the laser pulses. This is achieved by utilizing discharge chambers having a compact design. Compactly designed discharge chambers, however, tend to promote arcing which is not suitable for the generation of a laser beam and which damages the discharge chambers' electrodes.
Usually, the electrodes have a conductive structure comprising an extending ridge (sometimes referred to as a “nose portion”) and opposed shoulder portions. The protruding ridge portion is used to maintain the appropriate gap distance between the cathode and anode electrodes and to separate the bulk of the electrode bodies from each other. This separation can help to prevent arcing between, for example, shoulder portions of opposing electrodes, as well as from preionization pins to shoulder regions of the electrodes.
However, the use of a protruding ridge portion cannot guarantee the absence of arcing. Additionally, the protruding ridge portion can have a strong influence on the flows of the gas mixture between the electrodes. An optimized gas flow with higher gas speeds, such as on the order of about 30-50 m/s, can be necessary for a high repetition rate laser over 4 kHz. Due to unavoidable chemical reactions between the gas mixture and materials of the gas laser system as well as electrode burn-off, both causing solid and gaseous impurities, the gas mixture must be continuously cleaned by special gas filters. Furthermore, an optimized gas flow is necessary to lead away the excess heat produced during the laser beam generation process.
An improved approach that can be used to avoid arcing, while improving the gas flow between the electrodes, is disclosed in U.S. Pat. No. 7,079,565, assigned to the common assignee of this application. As described in this patent, a ceramic spoiler is placed at each shoulder region of an electrode, to act as an insulating barrier over the shoulder portion of the electrode. The outer surface of each ceramic spoiler has a shape that is optimized to improve the flow of gas between the electrodes. For mounting a ceramic spoiler to the electrode, the ceramic spoiler comprises a projecting tongue which is received in a channel of the electrode. The projecting tongue is biased against the sides of the channel by a spring mounted in the channel so that the ceramic spoiler is held in a stationary position with respect to the electrode.
One drawback to the use of ceramic spoilers of this type is that the ceramic spoilers have a certain minimal thickness due to the manufacturing process and mounts needed to connect the spoilers to the electrodes. Thus, the necessary minimal thickness of the ceramic spoilers sets a minimal limit for the dimensions of the discharge chamber. Furthermore, the necessity for a mount increases manufacture and material costs.
Accordingly, it would be desirable and highly advantageous to have an avenue to minimize the occurrence of arcing in a discharge chamber of a gas discharge laser system, and yet not to set a minimal limit for the discharge chambers' dimensions.
Therefore, an aspect of the present invention is to provide an electrode for a gas discharge laser system which minimizes the occurrence of arcing in the discharge chamber while not limiting the reduction of the discharge chambers' size.
A further aspect of the present invention is to provide an electrode for a gas discharge laser system which involves lower manufacture and material costs than an electrode having separate ceramic spoilers.
A still further aspect of the present invention is to provide an electrode for a gas discharge laser system which improves the preionization of the gas mixture volume and which is less subject to erosion, thereby allowing for a better long-term stability of the gas discharge.